Generating downlink frame and searching for cell

ABSTRACT

The present application relates to a method of generating a downlink frame. The method of generating the downlink frame includes: generating a first short sequence and a second short sequence indicating cell group information; generating a first scrambling sequence and a second scrambling sequence determined by the primary synchronization signal; generating a third scrambling sequence determined by the first short sequence and a fourth scrambling sequence determined by the second short sequence; scrambling the short sequences with the respective scrambling sequences; and mapping the secondary synchronization signal that includes the first short sequence scrambled with the first scrambling sequence, the second short sequence scrambled with the second scrambling sequence and the third scrambling sequence, the second short sequence scrambled with the first scrambling sequence and the first short sequence scrambled by the second scrambling sequence and the fourth scrambling sequence to a frequency domain.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Divisional patent applicationSer. No. 14/697,146, filed on Apr. 27, 2015, which is a divisional ofU.S. Continuation patent application Ser. No. 13/657,409, filed on Oct.22, 2012, which continuation of U.S. Continuation patent applicationSer. No. 12/488,272 filed on Jun. 19, 2009, which is a continuation ofPCT application No. PCT/KR2008/004223 filed on Jul. 18, 2008, whichclaims priority to and the benefit of, Korean Patent Application No.10-2007-0072837 filed on Jul. 20, 2007, Korean Patent Application No.10-2007-0083915 filed on Aug. 21, 2007, Korean Patent Application No.10-2008-0042907 filed on May 8, 2008, Korean Patent Application No.10-2008-0063388 filed on Jul. 1, 2008. The entire contents of theaforementioned applications are incorporated herein by reference.

BACKGROUND

(a) Field

The present application relates to a method of generating a downlinkframe and a method of searching for cells. More particularly, thepresent application relates to a method of generating a downlink frameand a method of searching for cells by using the downlink frame in anorthogonal frequency division multiplexing (OFDM)-based cellular system.

(b) Description of the Related Art

In a direct sequence code division multiple access (DS-CDMA) system, asequence hopping method is applied to a pilot channel so as to acquirecell synchronization and unique cell identification information.According to the sequence hopping method, a mobile station easilyperforms a cell search without a separating synchronization channel byintroducing a sequence hopping technology to the pilot channel. However,in the OFDM system, a number of channels that are capable of beingdistinguished by a frequency domain in a symbol duration of one timedomain is greater than that of those that are capable of beingdistinguished by a spread of CDMA in the symbol duration of one timedomain. Accordingly, when only the time domain is used, resources may bewasted in terms of capacity. For this reason, it is inefficient todirectly apply the sequence hopping method to the time domain of thepilot channel in the OFDM-based system. Therefore, it is preferable tosearch for the cell by efficiently using received signals in both timedomain and frequency domain.

An example of an existing technology for searching for a cell in theOFDM system includes a method that allocates synchronization informationand cell information by dividing one frame into four time blocks. Forthe above-described method, two frame structures have been proposed. Ina first frame structure, synchronization identification information,cell group identification information, and cell unique identificationinformation are allocated to four time blocks, respectively. In a secondframe structure, the synchronization identification information and thecell unique identification information are allocated to a first timeblock and a third time block, and the synchronization identificationinformation and the cell group identification information are allocatedto a second time block and a fourth time block.

According to the first frame structure, since the symbol synchronizationis acquired in only the first time block, it is impossible for themobile station to conduct rapid synchronization acquisition within aprescribed 5 ms during power-on or handover between heterogeneousnetworks. In addition, it is difficult to acquire diversity gain byaccumulating synchronization identification information so as to conductrapid synchronization acquisition.

According to the second frame structure, the unique cell identificationinformation or the cell group identification information is correlatedalong with the synchronization acquisition. Therefore, a cell searchingprocess is complex and a rapid cell search is difficult.

As an example of another technology for searching for the cell, a methodof acquiring the synchronization and searching for the cell by using aseparate preamble has been proposed. However, this method cannot beapplied to a system in which the preamble does not exist. Moreover, thepreamble is disposed in front of the frame. Accordingly, in a case inwhich the mobile station would like to acquire the synchronization at atime location that is not the start of the frame, there is a problem inthat it must wait for the next frame. Particularly, the mobile stationshould acquire initial symbol synchronization within 5 msec during thehandover among a GSM mode, a WCDMA mode, and a 3GPP LTE mode, but mayacquire the synchronization by a frame unit. For this reason, in somecases, the mobile station cannot acquire the initial symbolsynchronization within 5 msec.

As an example of another technology for searching for a cell, there is amethod of searching for the cell by allocating two short sequences to asecondary synchronization channel and by mapping cell ID information toa combination of two short sequences. According to this method, sinceinterference occurs between cells when the same short sequence isallocated to sectors adjacent to each other, there is a problem in thatperformance in searching cells is reduced.

SUMMARY

The present application has been made in an effort to provide a methodof generating a downlink frame that is capable of averaging interferencebetween sectors and a method of efficiently searching for cells byreceiving the downlink frame.

An exemplary embodiment provides a method of generating a downlinkframe, including: generating a first short sequence and a second shortsequence indicating cell group information; generating a firstscrambling sequence and a second scrambling sequence determined by theprimary synchronization signal; generating a third scrambling sequencedetermined by the first short sequence and a fourth scrambling sequencedetermined by the second short sequence; scrambling the first shortsequence with the first scrambling sequence and scrambling the secondshort sequence with the second scrambling sequence and the thirdscrambling sequence; scrambling the second short sequence with the firstscrambling sequence and scrambling the first short sequence with thesecond scrambling sequence and the fourth scrambling sequence; andmapping the secondary synchronization signal that includes the firstshort sequence scrambled with the first scrambling sequence, the secondshort sequence scrambled with the second scrambling sequence and thethird scrambling sequence, the second short sequence scrambled with thefirst scrambling sequence and the first short sequence scrambled withthe second scrambling sequence and the fourth scrambling sequence to afrequency domain.

Another embodiment provides an apparatus for generating a downlink frameincluding: a sequence generating unit that generates a first shortsequence and a second short sequence indicating cell group information,a first scrambling sequence and a second scrambling sequence determinedby the primary synchronization signal, a third scrambling sequencedetermined by the first short sequence and a fourth scrambling sequencedetermined by the second short sequence; and a synchronization signalgenerating unit that scrambles the first short sequence with the firstscrambling sequence and scrambles the second short sequence with thesecond scrambling sequence and the third scrambling sequence to generateone secondary synchronization signal, and scrambles the second shortsequence with the first scrambling sequence and scrambles the firstshort sequence with the second scrambling sequence and the fourthscrambling sequence to generate the other secondary synchronizationsignal.

Yet another embodiment provides a method of searching for a cell,including: receiving a downlink frame including a primarysynchronization signal and two secondary synchronization signal whichare different from each other; and estimating information of a cell byusing the primary synchronization signal and the two secondarysynchronization signal. In this case, in one secondary synchronizationsignal of the two secondary synchronization signal, a first shortsequence scrambled with a first scrambling sequence and a second shortsequence scrambled with a second scrambling sequence and a thirdscrambling sequence are alternately disposed on a plurality ofsub-carriers, in the other secondary synchronization signal of the twosecondary synchronization signal, a second short sequence scrambled witha first scrambling sequence and a first short sequence scrambled with asecond scrambling sequence and a fourth scrambling sequence arealternately disposed on a plurality of sub-carriers, and the first shortsequence and the second short sequence indicate cell group information,the first scrambling sequence and the second scrambling sequence aredetermined by the primary synchronization signal, the third scramblingsequence is determined by the first short sequence, and the fourthscrambling sequence is determined by the second short sequence.

Still another embodiment provides an apparatus for searching for a cell,including: a receiving unit that receives a downlink frame including aprimary synchronization signal and two secondary synchronization signalswhich are different from each other; a cell group estimating unit thatidentifies a cell group by using the two secondary synchronizationsignal; and a cell estimating unit that identifies a cell in the cellgroup by using the primary synchronization signal. In this case, in onesecondary synchronization signal of the two secondary synchronizationsignal, a first short sequence scrambled with a first scramblingsequence and a second short sequence scrambled with a second scramblingsequence and a third scrambling sequence are alternately disposed on aplurality of sub-carriers, and in the other secondary synchronizationsignal of the two secondary synchronization signal, a second shortsequence scrambled with a first scrambling sequence and a first shortsequence scrambled with a second scrambling sequence and a fourthscrambling sequence are alternately disposed on a plurality ofsub-carriers, and the first short sequence and the second short sequenceindicate cell group information, the first scrambling sequence and thesecond scrambling sequence are determined by the primary synchronizationsignal, the third scrambling sequence is determined by the first shortsequence and the fourth scrambling sequence is determined by the secondshort sequence.

Still another embodiment provides a recording medium that records aprogram for executing the method of generating the downlink frame. Therecording medium records a program including: generating a first shortsequence and a second short sequence indicating cell group information;generating a first scrambling sequence and a second scrambling sequencedetermined by the primary synchronization signal; generating a thirdscrambling sequence determined by the first short sequence and a fourthscrambling sequence determined by the second short sequence; scramblingthe first short sequence with the first scrambling sequence andscrambling the second short sequence with the second scrambling sequenceand the third scrambling sequence; scrambling the second short sequencewith the first scrambling sequence and scrambling the first shortsequence with the second scrambling sequence and the fourth scramblingsequence; and mapping the secondary synchronization signal that includesthe first short sequence scrambled with the first scrambling sequenceand the second short sequence scrambled with the second scramblingsequence and the third scrambling sequence, the second short sequencescrambled with the first scrambling sequence and the first shortsequence scrambled with the second scrambling sequence and the fourthscrambling sequence to a frequency domain.

According to the above-mentioned embodiment, interference betweensectors can be reduced by scrambling the short sequences due to thescrambling sequences, thereby improving performance for searching forcells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a downlink frame in an OFDM systemaccording to an exemplary embodiment.

FIG. 2 is a diagram illustrating a configuration of a secondarysynchronization channel when two sequences are mapped to a frequencydomain in a localization form.

FIG. 3 is a diagram illustrating a configuration of a secondarysynchronization channel when two sequences are mapped to a frequencydomain in a distribution form.

FIG. 4 is a block diagram of an apparatus for generating a downlinkframe according to the exemplary embodiment.

FIG. 5 is a flowchart illustrating a method of generating a downlinkframe according to the exemplary embodiment.

FIG. 6 is a diagram illustrating a first method of generating asecondary synchronization signal according to the exemplary embodiment.

FIG. 7 is a diagram illustrating a second method of generating asecondary synchronization signal according to the exemplary embodiment.

FIG. 8 is a diagram illustrating a third method of generating asecondary synchronization signal according to the exemplary embodiment.

FIG. 9 is a block diagram of an apparatus for searching for cellsaccording to an exemplary embodiment.

FIG. 10 is a flowchart illustrating a method of searching for a cellaccording to a first exemplary embodiment.

FIG. 11 is a flowchart illustrating a method of searching for a cellaccording to a second exemplary embodiment.

DETAILED DESCRIPTION

In the following detailed description, only certain exemplaryembodiments have been shown and described, simply by way ofillustration. As those skilled in the art would realize, the describedembodiments may be modified in various different ways, all withoutdeparting from the spirit or scope of the present application. Inaddition, parts that are irrelevant to the description of the presentapplication are omitted in the drawings, to clarify the invention. Likereference numerals designate like elements throughout the specification.

Throughout the specification, unless explicitly described to thecontrary, the word “comprise” and variations such as “comprises” or“comprising” will be understood to imply the inclusion of statedelements but not the exclusion of any other elements. In addition, theterm “unit” described in the specification means a unit for processingat least one function and operation, and can be implemented by hardwarecomponents or software components and combinations thereof.

First, referring to FIGS. 1 to 3, a downlink frame of an OFDM system anda configuration of a synchronization channel according to an exemplaryembodiment will be described.

FIG. 1 is a diagram illustrating a downlink frame of an OFDM systemaccording to an exemplary embodiment. In FIG. 1, a horizontal axisrepresents a time axis and a vertical axis represents a frequency axisor sub-carrier axis.

As shown in FIG. 1, a downlink frame 110 according to the exemplaryembodiment has a time duration of 10 msec and includes ten sub-frames120. Each sub-frame 120 has a time duration of 1 msec and includes twoslots 130. Each slot 130 includes six or seven OFDM symbols. The lengthof a cyclic prefix in a case in which one slot includes six symbols isgreater than that of a cyclic prefix in a case in which one slotincludes seven symbols.

As shown in FIG. 1, the downlink frame 110 according to the exemplaryembodiment includes two synchronization durations 140 in total,including synchronization durations 140 in slot No. 0 and slot No. 10,respectively. However, it is not necessarily limited thereto. Thedownlink frame 110 may include a synchronization duration in any slot,and may include one synchronization duration or three or moresynchronization durations. Since the length of the cyclic prefix may bedifferent in each slot, it is preferable that the synchronizationduration is located at an end of the slot.

Each slot includes a pilot duration.

The synchronization duration according to the exemplary embodimentincludes a primary synchronization channel and a secondarysynchronization channel, and the primary synchronization channel and thesecondary synchronization channel are disposed so as to be adjacent toeach other in view of time. As shown in FIG. 1, the primarysynchronization channel is located at the end of the slot, and thesecondary synchronization channel is located right ahead of the primarysynchronization channel.

The primary synchronization channel includes a primary synchronizationsignal having information for identifying symbol synchronization andfrequency synchronization, and some information for cellidentification(ID). The secondary synchronization channel includes asecondary synchronization signal having remaining information for thecell ID, and information for identifying frame synchronization. A mobilestation identifies the cell ID of cell by combining the cell IDinformation of the primary synchronization channel and the cell IDinformation of the secondary synchronization channel.

For instance, assuming that the total number of cell IDs is 510, ifthree identification sequences are allocated to the primarysynchronization channel to divide all 510 cell IDs into three groups andif 170 sequences are allocated to the secondary synchronization channel(3×170=510), the information on all of the 510 cell IDs can berepresented.

Another method is that the 510 cell IDs are divided into 170 groups byusing 170 secondary synchronization signals that are allocated to thesecondary synchronization channel, and information on cell IDs in eachcell group can be represented by three primary synchronization signalsthat are allocated to the primary synchronization channel.

Since the secondary synchronization channel includes the information foridentifying the frame synchronization as well as information for thecell ID, two secondary synchronization channels included in one frameare different from each other.

FIG. 2 is a diagram illustrating a configuration of a secondarysynchronization channel when two short sequences are mapped to afrequency domain in a localization form, and FIG. 3 is a diagramillustrating a configuration of a secondary synchronization channel whentwo short sequences are mapped to a frequency domain in a distributionform.

Referring to FIG. 2 to FIG. 3, a secondary synchronization signal, whichis inserted into a secondary synchronization channel, according to anexemplary embodiment is formed by combining two short sequences. Cellgroup information and frame synchronization information are mapped tothe two short sequences.

As shown in FIG. 2, a first short sequence may be locally allocated tosub-carriers, and then the second short sequence may be locallyallocated to remaining sub-carriers. In addition, as shown in FIG. 3,the first short sequence may be allocated to every even-numberedsub-carriers (n=0, 2, 4, . . . , 60), and the second short sequence maybe allocated to every odd-numbered sub-carrier (n=1, 3, 5, . . . , 61).

The short sequence length corresponds to half of the number ofsub-carriers allocated to the secondary synchronization channel. Thatis, the number of short sequence elements that can be generated is up tohalf of the number of sub-carriers allocated to the secondarysynchronization channel. For instance, when the number of sub-carriersallocated to the secondary synchronization channel is 62, the shortsequence length corresponds to 31 and the number of short sequenceelements that can be generated is up to 31.

Since two short sequences are allocated to each secondarysynchronization channel, the number of secondary synchronizationsequences generated by a combination of two short sequences is 961(=31×31) at maximum. However, since the information that should beincluded in the secondary synchronization channel is cell groupinformation and frame boundary information, 170 or 340 (=170×2)secondary synchronization sequences are required. Accordingly, thenumber 961 is a sufficiently large value in comparison with the number170 or 340.

Next, an apparatus for generating a downlink frame according to anexemplary embodiment will be described with reference to FIG. 4. FIG. 4is a block diagram of the apparatus for generating the downlink frameaccording to the exemplary embodiment.

As shown in FIG. 4, the apparatus for generating the downlink frameaccording to the exemplary embodiment of includes a sequence generatingunit 410, a synchronization signal generating unit 420, a frequencymapping unit 430, and an OFDM transmitting unit 440.

The sequence generating unit 410 generates a sequence for acquiring timeand frequency synchronization, a cell identification sequence, aplurality of short sequences, and a scrambling sequence for reducingadjacent cell interference, respectively, and transmits them to thesynchronization signal generating unit 420.

The synchronization signal generating unit 420 generates a primarysynchronization signal, a secondary synchronization signal, and a pilotpattern by using sequences received from the sequence generating unit410.

The synchronization signal generating unit 420 generates the primarysynchronization signal by using the sequence for acquiring time andfrequency synchronization and the cell identification sequence. Inaddition, the synchronization signal generating unit 420 generates thesecondary synchronization signal by using the plurality of shortsequences and the scrambling sequences for reducing adjacent cellinterference.

The synchronization signal generating unit 420 generates the pilotpattern of downlink signals by allocating a unique scrambling sequenceallocated to each cell for encoding a common pilot symbol and datasymbol of a cellular system to the pilot channel.

The frequency mapping unit 430 generates the downlink frame by mappingthe primary synchronization signal, the secondary synchronizationsignal, and the pilot pattern that are generated from thesynchronization signal generating unit 420 and frame control informationand transmission traffic data that are transmitted from external sourcesto the time and frequency domains.

The OFDM transmitting unit 440 receives the downlink frame from thefrequency mapping unit 430 and transmits the downlink frame throughgiven transmission antenna.

Referring to FIG. 5 to FIG. 8, a method of generating a downlink frameaccording to an exemplary embodiment will be described. FIG. 5 is aflowchart illustrating the method of generating the downlink frameaccording to the exemplary embodiment.

As shown in FIG. 5, the sequence generating unit 410 generates aplurality of short sequences and a plurality of scrambling sequences forreducing interference of a plurality of adjacent cells and transmitsthem to the synchronization signal generating unit 420 (S510).

The synchronization signal generating unit 420 generates a secondarysynchronization signal by using the short sequences and the scramblingsequences for reducing interference of the plurality of adjacent cellsreceived from the sequence generating unit 410 (S520). In the exemplaryembodiment, it is described that one frame includes two secondarysynchronization channels. However, it is not limited thereto.

Referring to FIG. 6 to FIG. 8, three different methods of generating asecondary synchronization signal according to an exemplary embodimentwill be described. FIG. 6 is a diagram illustrating the first method ofgenerating a secondary synchronization signal according to the exemplaryembodiment, FIG. 7 is a diagram illustrating the second method ofgenerating a secondary synchronization signal according to the exemplaryembodiment, and FIG. 8 is a diagram illustrating the third method ofgenerating a secondary synchronization signal according to the exemplaryembodiment.

A short sequence (wn) is a binary sequence (or binary code) representingcell group information. That is, the short sequence (wn) is the binarysequence allocated to a cell group number and frame synchronization.Moreover, the length of the short sequence corresponds to half of thenumber of sub-carriers allocated to the secondary synchronizationchannel. In the exemplary embodiment, it is described that the number ofsub-carriers allocated to the secondary synchronization channel is 62.However, it is not limited thereto. Accordingly, the short sequencelength according to the exemplary embodiment is 31.

The first short sequence w0 is allocated to even-numbered sub-carriersof the first secondary synchronization channel and is defined as givenin Equation 1.

w0=[w0(0),w0(1), . . . ,w0(k), . . . ,w0(30)]  (Equation 1)

Here, k denotes an index of the even-numbered sub-carriers used for asecondary synchronization channel.

The second short sequence w1 is allocated to odd-numbered sub-carriersof the first secondary synchronization channel and is defined as givenin Equation 2.

w1=[w1(0),w1(1), . . . ,w1(m), . . . ,w1(30)]  (Equation 2)

Here, m denotes an index of the odd-numbered sub-carriers used for thesecondary synchronization channel.

The third short sequence w2 is allocated to even-numbered sub-carriersof the second secondary synchronization channel and is defined as givenin Equation 3.

w2=[w2(0),w2(1), . . . ,w2(k),w2(30)]  (Equation 3)

The fourth short sequence w3 is allocated to odd-numbered sub-carriersof the second secondary synchronization channel and is defined as givenin Equation 4.

w3=[w3(0),w3(1), . . . ,w3(m), . . . ,w3(30)]  (Equation 4)

Here, the short sequences w0, w1, w2, and w3 may be different sequences.In addition, the relationship between the short sequences w0, w1, w2,and w3 may be represented as w0=w3 and w1=w2 (or w0=w2 and w1=w3). Giventhat w0=w3 and w1=w2, then the pattern of short sequences allocated tothe second secondary synchronization channel can be determined onlythrough the pattern of short sequences allocated to the first secondarysynchronization channel. Accordingly, by storing only 170 secondarysynchronization sequences generated by a combination of two shortsequences allocated to the first secondary synchronization channel, amobile station can reduce the complexity needed to obtaining the cellgroup information and frame boundary information.

According to the first method of generating a secondary synchronizationsignal as shown in FIG. 6, the first short sequence is allocated toevery even-numbered sub-carrier of the first secondary synchronizationchannel and the second short sequence is allocated to every odd-numberedsub-carrier of the first secondary synchronization channel. In addition,the third short sequence is allocated to every even-numbered sub-carrierof the second secondary synchronization channel and the fourth shortsequence is allocated to every odd-numbered sub-carrier of the secondsecondary synchronization channel.

According to the first method of generating the secondarysynchronization signal, the secondary synchronization signal is formedby a combination of two short sequences having the length of 31.Accordingly, the number of secondary synchronization signals is 961which is a sufficiently large value in comparison with the number 170 or340.

According to the second method of generating the secondarysynchronization signal shown in FIG. 7, a first sequence determined byEquation 5 is allocated to every even-numbered sub-carrier of the firstsecondary synchronization channel (slot 0), and a second sequencedetermined by Equation 6 is allocated to every odd-numbered sub-carrierof the first secondary synchronization channel (slot 0). In addition, athird sequence determined by Equation 7 is allocated to everyeven-numbered sub-carrier of the second secondary synchronizationchannel (slot 10), and a fourth sequence determined by Equation 8 isallocated to every odd-numbered sub-carrier of the second secondarysynchronization channel (slot 10).

A scrambling sequence P_(j,0,1) scrambling the first short sequence w0is defined by P_(j,0,1)=[P_(j,0,1)(0), P_(j,0,1)(1), . . . ,P_(j,0,1)(k), . . . , P_(j,0,1)(30)], where j (j=0, 1, 2) is the numberof the cell identification sequence allocated to the primarysynchronization channel. Accordingly, the scrambling sequence P_(j,0,1)is determined by the primary synchronization signal. The scramblingsequence P_(j,0,1) is a known value when a sequence is demapped to finda cell ID group and a frame boundary in the mobile station.

As indicated in Equation 5, each element of a first sequence c₀according to the second method of generating the secondarysynchronization signal is a product of each element of the first shortsequence w0 and each element of the scrambling sequence P_(j,0,1)corresponding thereto.

c ₀ =[w0(0)P _(j,0,1)(0),w0(1)P _(j,0,1)(1), . . . ,w0(k)P _(j,0,1)(k),. . . ,w0(30)P _(j,0,1)(30)]  (Equation 5)

Here, k denotes an index of the even-numbered sub-carriers used for thesecondary synchronization channel.

The scrambling sequence scrambling the second short sequence w1 isP_(j,1,1) and S_(w0).

The scrambling sequence P_(j,1,1) is P_(j,1,1)=[P_(j,1,1)(0),P_(j,1,1)(1), . . . , P_(j,1,1)(m), . . . , P_(j,1,1)(30)], where j(j=0, 1, 2) is the number of the cell identification sequence allocatedto the primary synchronization channel. Accordingly, the scramblingsequence P_(j,1,1) is determined by the primary synchronization signal.In addition, the scrambling sequence P_(j,1,1) may be the same as thescrambling sequence P_(j,0,1) or may be different from the scramblingsequence P_(j,0,1). When the scrambling sequence P_(j,1,1) is differentfrom the scrambling sequence P_(j,0,1), it can be possible to reduceinterference.

The scrambling sequence P_(j,1,1) is a previously known value when asequence is demapped to find a cell ID group and a frame boundary in themobile station.

In addition, the scrambling sequence S_(w0) is S_(w0)=[S_(w0)(0),S_(w0)(1), . . . , S_(w0)(m), . . . , S_(w0)(30)], and the scramblingsequence S_(w0) is determined by the first short sequence w0.

At this time, a plurality of short sequences are grouped into aplurality of short sequence group and the S_(w0) may be determined by ashort sequence group to which the first short sequence is assigned bygrouping short sequences.

For example, according to the exemplary embodiment, since the length ofthe first short sequence is 31, there are 31 short sequences.Accordingly, by assigning the short sequences Nos. 0-7 to the group 0,the short sequences Nos. 8-15 to the group 1, the short sequences Nos.16-23 to the group 2, and the short sequences Nos. 24-30 to the group 3.Accordingly S_(w0) is determined by mapping a length-31 scrambling codeto the group to which the first short sequence number is assigned.

Furthermore, 31 short sequences may be classified into eight groups bygrouping the numbers of the first short sequences having the identicalremainder when we divide each number of short sequences by 8. That is,by assigning the short sequence number having the remainder of 0 whendividing the short sequence numbers by 8 to the group 0, the shortsequence having the remainder of 1 when dividing the short sequencenumbers by 8 to the group 1, the short sequence having the remainder of2 when dividing the short sequence numbers by 8 to the group 2, theshort sequence having the remainder of 3 when dividing the shortsequence numbers by 8 to the group 3, the short sequence having theremainder of 4 when dividing the short sequence numbers by 8 to thegroup 4, the short sequence having the remainder of 5 when dividing theshort sequence numbers by 8 to the group 5, the short sequence havingthe remainder of 6 when dividing the short sequence numbers by 8 to thegroup 6, and the short sequence having the remainder of 7 when dividingthe short sequence numbers by 8 to the group 7. Accordingly S_(w0) isdetermined by mapping a length-31 scrambling code to the group to whichthe first short sequence number is assigned.

As indicated in Equation 6, each element of a second sequence c₁according to the second method of generating the secondarysynchronization signal is a product of each element of the second shortsequence w1 and each element of the scrambling sequences P_(j,1,1) andS_(w0) corresponding thereto.

c ₁ =[w1(0)S _(w0)(0)P _(j,1,1)(0),w1(1)S _(w0)(1)P _(j,1,1)(1), . . .,w1(m)S _(w0)(m)P _(j,1,1)(m), . . . ,w1(30)S _(w0)(30)P_(j,1,1)(30)]  (Equation 6)

Herein, m denotes the index of odd-numbered sub-carriers used for thesecondary synchronization channel.

A scrambling sequence P_(j,0,2) for scrambling a third short sequence w2is P_(j,0,2)=[P_(j,0,2)(0), P_(j,0,2)(1), . . . , P_(j,0,2)(k) . . . ,P_(j,0,2)(30)], where j (j=0, 1, 2) is the number of the cellidentification sequence allocated to the primary synchronizationchannel. Accordingly, the scrambling sequence P_(j,0,2) is determined bythe primary synchronization signal. In addition, the scrambling sequenceP_(j,0,2) is a previously known value when the sequence is demapped tofind the cell ID group and frame boundary in the mobile station.

As indicated in Equation 7, each element of a third sequence c₂according to the second method of generating the secondarysynchronization signal is a product of each element of the third shortsequence w2 and each element of the scrambling sequence P_(j,0,2)corresponding thereto.

c ₂ =[w2(0)P _(j,0,2)(0),w2(1)P _(j,0,2)(1), . . . ,w2(k)P _(j,0,2)(k),. . . ,w2(30)P _(j,0,2)(30)]  (Equation 7)

Herein, k denotes the index of even-numbered sub-carriers used for thesecondary synchronization channel.

Scrambling sequences for scrambling a fourth short sequence areP_(j,1,2) and S_(w2).

The scrambling sequence P_(j,1,2) is P_(j,1,2)=[P_(j,1,2)(0),P_(j,1,2)(1), . . . , P_(j,1,2)(m), . . . , P_(j,1,2)(30)], and j (j=0,1, 2) is the number of the cell identification sequence allocated to theprimary synchronization channel. Accordingly, the scrambling sequenceP_(j,1,2) is determined by the primary synchronization signal. Thescrambling sequence P_(j,1,2) is a previously known value when asequence is demapped to find the cell ID group and frame boundary in themobile station.

Furthermore, the scrambling sequence S_(w2) is S_(w2)=[S_(w2)(0),S_(w2)(1), S_(w2)(m), . . . , S_(w2)(30)], and the scrambling sequenceS_(w2) is determined by the third short sequence w2.

At this time, the S_(w2) may be determined by a short sequence group towhich the third short sequence is assigned by grouping short sequences.

For example, according to the exemplary embodiment, since the length ofthe third short sequence is 31 as well, there are 31 short sequences.Accordingly, by assigning the short sequences Nos. 0-7 to the group 0,the short sequences Nos. 8-15 to the group 1, the short sequences Nos.16-23 to the group 2, and the short sequences Nos. 24-30 to the group 3.Accordingly S_(w2) is determined by mapping a length-31 scrambling codeto the group to which the third short sequence number is assigned.

Furthermore, 31 short sequences may be classified into eight groups bygrouping the numbers of the third short sequences having the identicalremainder when we divide each number of short sequences by 8. That is,by assigning the short sequence number having the remainder of 0 whendividing the short sequence numbers by 8 to the group 0, the shortsequence having the remainder of 1 when dividing the short sequencenumbers by 8 to the group 1, the short sequence having the remainder of2 when dividing the short sequence numbers by 8 to the group 2, theshort sequence having the remainder of 3 when dividing the shortsequence numbers by 8 to the group 3, the short sequence having theremainder of 4 when dividing the short sequence numbers by 8 to thegroup 4, the short sequence having the remainder of 5 when dividing theshort sequence numbers by 8 to the group 5, the short sequence havingthe remainder of 6 when dividing the short sequence numbers by 8 to thegroup 6, and the short sequence having the remainder of 7 when dividingthe short sequence numbers by 8 to the group 7. Accordingly S_(w2) isdetermined by mapping a length-31 scrambling code to the group to whichthe third short sequence number is assigned.

As indicated in Equation 8, each element of a fourth sequence c₃according to the second method of generating the secondarysynchronization signal is a product of each element of the fourth shortsequence w3 and each element of the scrambling sequences P_(j,1,2) andS_(w2) corresponding thereto.

c ₃ =[w3(0)S _(w2)(0)P _(j,1,2)(0),w3(1)S _(w2)(1)P _(j,1,2)(1), . . .,w3(m)S _(w2)(m)P _(j,1,2)(m), . . . ,w3(30)S _(w2)(30)P_(j,1,2)(30)]  (Equation 8)

Herein, m denotes the index of odd-numbered sub-carriers used for thesecondary synchronization channel.

Here, the relationship between the scrambling sequences and the shortsequences may be set as P_(j,0,1)=P_(j,0,2), P_(j,1,1)=P_(j,1,2),P_(j,0,1)≠P_(j,1,1), P_(j,0,2)≠P_(j,1,2), and w0≠w1≠w2≠w3 (or w0=w3 andw1=w2). In this case, the cell group and frame identify information aremapped to the combination of the first to fourth short sequences, andthe number of descrambling hypotheses in the mobile station with respectto the scrambling of secondary synchronization channel determined by thecell identification sequence number of the primary synchronizationchannel is reduced to 3.

Furthermore, the relationship between the scrambling sequences and theshort sequences may be set as P_(j,0,1)≠P_(j,0,2), P_(j,1,1)≠P_(j,1,2),P_(j,0,1)≠P_(j,1,1), P_(j,0,2)≠P_(j,1,2), w0=w2, and w1=w3. In thiscase, the cell group information is mapped to the combination of thefirst short sequence and the second short sequence, and the framesynchronization information is mapped to the scrambling sequences(P_(j,0,1), P_(j,0,2), P_(j,1,1), P_(j,1,2)) of the secondarysynchronization channel determined by the cell identification sequencenumber of the primary synchronization channel. Then, the number ofdescrambling hypotheses of the mobile station with respect to thescrambling of the secondary synchronization channel determined by thecell identification sequence number of the primary synchronizationchannel is increased to 6. However, the combination number of the cellgroup identification sequences is reduced to half, and the number ofdescrambling hypotheses of the mobile station with respect to thescrambling determined by the first and third short sequences is alsoreduced to half.

As shown in FIG. 8, in the third method of generating a secondarysynchronization signal, a first sequence determined by Equation 9 isallocated to every even-numbered sub-carrier of a first secondarysynchronization channel, and a second sequence determined by Equation 10is allocated to every odd-numbered sub-carrier of the first secondarysynchronization channel. Moreover, a third sequence determined byEquation 11 is allocated to every even-numbered sub-carrier of a secondsecondary synchronization channel, and a fourth sequence determined byEquation 12 is allocated to every odd-numbered sub-carrier of the secondsecondary synchronization channel.

That is, according to the second method of generating the secondarysynchronization signal, the first short sequence is scrambled with afirst scrambling sequence having the length of 31, which is determinedby the cell identification sequence allocated to the primarysynchronization channel, and the second short sequence is scrambled witha second scrambling sequence having the length of 31, which isdetermined by the cell identification sequence allocated to the primarysynchronization channel. However, according to the third method ofgenerating the secondary synchronization signal, the first shortsequence and the second short sequence are scrambled with a scramblingsequence having the length of 62, which is determined by the cellidentification sequence allocated to the primary synchronizationchannel.

P_(j,1) is the scrambling sequence that scrambles the first shortsequence and the second short sequence, and P_(j,2) is the scramblingsequence that scrambles the third short sequence and the fourth shortsequence. The scrambling sequences P_(j,1) and P_(j,2) are representedas P_(j,1)=[P_(j,1)(0), P_(j,1)(1), . . . , P_(j,1)(k), . . . ,P_(j,1)(61)], and P_(j,2)=[P_(j,2)(0), P_(j,2)(1), . . . , P_(j,2)(k), .. . , P_(j,2)(61)].

Here, j (j=0, 1, 2) is the number of the cell identification sequenceallocated to the primary synchronization channel. Accordingly, thescrambling sequences P_(j,1) and P_(j,2) are determined by the number ofthe cell identification sequence allocated to the primarysynchronization channel.

According to the third method of generating the secondarysynchronization signal, the first sequence c₀ is as indicated inEquation 9, the second sequence c₁ is as indicated in Equation 10, thethird sequence c₂ is as indicated in Equation 11, and the fourthsequence c₃ is as indicated in Equation 12.

c ₀ =[w0(0)P _(j,1)(0),w0(1)P _(j,1)(1), . . . ,w0(k)P _(j,1)(k), . . .,w0(30)P _(j,1)(30)]  (Equation 9)

c ₁ =[w1(0)S _(w0)(0)P _(j,1)(31), . . . ,w1(1)S _(w0)(1)P_(j,1)(32),w1(m)S _(w0)(m)P _(j,1)(31+m), . . . ,w1(30)S _(w0)(30)P_(j,1)(61)]  (Equation 10)

c ₂ =[w2(0)P _(j,2)(0),w2(1)P _(j,2)(1), . . . ,w2(k)P _(j,2)(k), . . .,w2(30)P _(j,2)(30)]  (Equation 11)

C ₃ =[w3(0)S _(w2)(0)P _(j,2)(31),w3(1)S _(w2)(1)P _(j,2)(32), . . .,w3(m)S _(w2)(m)P _(j,2)(31+m), . . . ,w3(30)S _(w2)(30)P_(j,2)(61)]  (Equation 12)

In Equation 9 to Equation 12, k denotes the index of the even-numberedsub-carriers to be used for the secondary synchronization channel, and mdenotes the index of the odd-numbered sub-carriers to be used for thesecondary synchronization channel.

The frequency mapping unit 430 generates the downlink frame by mappingthe secondary synchronization signal that are generated from thesynchronization signal generating unit 420, and transmission trafficdata to the time and frequency domains S530.

The OFDM transmitting unit 440 receives the downlink frame from thefrequency mapping unit 430 and transmits the downlink frame throughgiven transmission antenna S540.

A method of searching for cells by the mobile station by using thedownlink frame generated by the exemplary embodiment will now bedescribed with reference to FIG. 9 and FIG. 11.

FIG. 9 is a block diagram of an apparatus for searching for cellsaccording to the exemplary embodiment, FIG. 10 is a flowchartillustrating a cell searching method according to a first exemplaryembodiment, and FIG. 11 is a flowchart illustrating a cell searchingmethod according to a second exemplary embodiment.

As shown in FIG. 9, the apparatus for searching for the cells accordingto the exemplary embodiment includes a receiving unit 710, a symbolsynchronization estimating and frequency offset compensating unit 720, aFourier transforming unit 730, and a cell ID estimating unit 740.

A cell searching method according to the first exemplary embodiment willnow be described with reference to FIG. 10.

As shown in FIG. 10, the receiving unit 710 receives the framestransmitted from the base station, and the symbol synchronizationestimating and frequency offset compensating unit 720 filters thereceived signal by as much as a bandwidth allocated to thesynchronization channel and acquires the symbol synchronization byrespectively correlating the filtered received signal and a plurality ofknown primary synchronization signals, and compensates the frequencyoffset by estimating frequency synchronization (S810). The symbolsynchronization estimating and frequency offset compensating unit 720respectively correlates the filtered received signal and the pluralityof known primary synchronization signals and estimates a time of thelargest correlation value as the symbol synchronization, and transmits anumber of a primary synchronization signal having the largestcorrelation value to the cell ID estimating unit 740. At this time, thefrequency offset may be compensated in the frequency domain afterperforming the Fourier transform.

The Fourier transforming unit 730 performs Fourier transform of thereceived signals on the basis of the symbol synchronization estimated bythe symbol synchronization estimating and frequency offset compensatingunit 720 (S820).

The cell ID estimating unit 740 estimates a cell ID group and framesynchronization by respectively correlating the Fourier transformedreceived signal with a plurality of known secondary synchronizationsignals S830. The cell ID estimating unit 740 respectively correlates aplurality of secondary synchronization signals with the Fouriertransformed received signal, and estimates the frame synchronization andthe cell ID group by using a secondary synchronization signal that hasthe largest correlation value. Herein, the plurality of secondarysynchronization signals are given by applying P_(j,0,1), P_(j,0,2),P_(j,1,1) and P_(j,1,2) that are determined in accordance with a primarysynchronization signal that corresponds to the number of a primarysynchronization signal transmitted from the symbol synchronizationestimating and frequency offset compensating unit 720 to Equation 5 toEquation 8, At this time, in the case that a synchronization channelsymbol exists in one slot or one OFDM symbol within one frame, thesymbol synchronization becomes frame synchronization, and therefore, itis not necessary to additionally acquire frame synchronization.

In addition, the cell ID estimating unit 740 estimates cell IDs by usingthe number of a primary synchronization signal transmitted from thesymbol synchronization estimating and frequency offset compensating unit720 and the estimated cell ID group S840. At this time, the cell IDestimating unit 740 estimates the cell ID with reference to a knownmapping relationship between cell ID, the cell ID group, and a number ofprimary synchronization signal.

The estimated cell ID information may be verified by using scramblingsequence information included in the pilot symbol duration.

A cell searching method according to the second exemplary embodimentwill now be described with reference to FIG. 11.

As shown in FIG. 11, the receiving unit 710 receives a frame transmittedfrom the base station, and the symbol synchronization estimating andfrequency offset compensating unit 720 filters the received signal by asmuch as a bandwidth allocated to the synchronization channel andacquires the symbol synchronization by respectively correlating thefiltered received signal and a plurality of known primarysynchronization signals, and compensates the frequency offset byestimating frequency synchronization S910. The symbol synchronizationestimating and frequency offset compensating unit 720 respectivelycorrelates the filtered received signal and the plurality of knownprimary synchronization signals and estimates a time of the largestcorrelation value as the symbol synchronization, and transmits aplurality of correlation values of the plurality of known primarysynchronization signals and filtered received signal to the cell IDestimating unit 740. At this time, the frequency offset compensation maybe performed in the frequency domain after Fourier-transformed.

The Fourier transforming unit 730 Fourier-transforms the received signalwith reference to the symbol synchronization that is estimated by thesymbol synchronization estimating and frequency offset compensating unit720 S920.

The cell ID estimating unit 740 estimates cell IDs by using theplurality of correlation values transmitted from the symbolsynchronization estimating and frequency offset compensating unit 720,and correlation values of the Fourier-transformed received signal and aplurality of known secondary synchronization signals S930. The cell IDestimating unit searches a secondary synchronization signal having thelargest correlation value by correlating each of the plurality of knownsecondary synchronization signals with the Fourier-transformed receivedsignal for each of the plurality of known primary synchronizationsignals. Here, the plurality of secondary synchronization signals aregiven by applying P_(j,0,1), P_(j,0,2), P_(j,1,1) and P_(j,1,2) that aredetermined in accordance with the corresponding primary synchronizationsignal to Equation 5 to Equation 8.

In addition, the cell ID estimating unit 740 combines the correlationvalue of each known primary synchronization signal transmitted from thesymbol synchronization estimating and frequency offset compensating unit720 and the correlation value of the secondary synchronization signalhaving the largest correlation value for each of the plurality of knownprimary synchronization signals.

The cell ID estimating unit 740 estimates frame synchronization and acell ID group by using a secondary synchronization signal having thelargest combined value among the combined values of the correlationvalues of a primary synchronization signal and a secondarysynchronization signal. In addition, the cell ID estimating unit 740estimates a cell ID by using the primary synchronization signal havingthe largest combined value and the estimated cell ID group. At thistime, the cell ID estimating unit 740 estimates the cell ID withreference to a known mapping relationship between the cell ID group,cell ID and the primary synchronization signal number.

The exemplary embodiment can be not only implemented by theabove-described apparatus and/or method, but can be implemented by, forexample, a program that achieves the function corresponding to theconfiguration of the exemplary embodiment and a recording medium inwhich the program is recorded. This will be easily implemented from theabove-described exemplary embodiment by those skilled in the relatedart. Examples of the recording medium may include, but not limited to, aread only memory (ROM), a random access memory (RAM), an electricallyprogrammable read-only memory (EEPROM), a flash memory, etc. The programmay be executed by one or more hardware processors to achieve thefunction corresponding to the configuration of the exemplary embodiment.Examples of the hardware processor may include, but not limited to, aDSP (digital signal processor), a CPU (central processing unit), an ASIC(application specific integrated circuit), a programmable logic element,such as an FPGA (field programmable gate array), etc.

While this application has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A method of generating a down link frame in abase station, comprising: generating a downlink frame including aprimary synchronization signal and a secondary synchronization signal;and including cell identity group information in the secondarysynchronization signal and including cell identity information within acell identity group in the primary synchronization signal so that aterminal searches for a cell using the cell identity group informationin the secondary synchronization signal and the cell identityinformation in the primary synchronization signal, wherein the secondarysynchronization signal comprises a first short sequence and a secondshort sequence, the first short sequence is scrambled with a firstscrambling sequence, and the second short sequence is scrambled with asecond scrambling sequence and a third scrambling sequence, wherein thefirst scrambling sequence and the second scrambling sequence aredetermined based on the cell identity information contained in theprimary synchronization signal, and the third scrambling sequence isdetermined based on the first short sequence.